Conventional solar thermal energy systems use light to generate heat, and such systems operate in a similar manner. These systems have a surface by which the energy of the Sun is absorbed, such as simple hot water heaters, or a surface that reflects the solar energy to a secondary point where it is absorbed or, in the case of concentrated solar, lenses or mirrors are used to increase the Sun's intensity by focusing large amounts of energy on a small absorptive surface. This is the case with solar troughs, parabolic dishes, solar towers, and variations in between.
However, conventional systems only reflect visible light and some of the ultraviolet (UV) spectrum in significant quantities, which account for only 47% and 2%, respectively, of the solar energy that reaches the Earth. Further, visible light and UV light are unreliable sources that are only available in useful quantities during the day and when clouds, fog, haze, and other such blocking bodies are not obstructing the Sun.